African swine fever (ASF)

Overview ASF is caused by the African swine fever virus (ASFV). ASFV infects domestic pigs, warthogs, and bushpigs. Transmission occurs through direct contact between healthy and sick animals, or indirect contact through infected feed, as well as through biological vectors (soft ticks).

The disease is endemic in the southern hemisphere of Africa, as well as on the Iberian Peninsula and in Sardinia. Sporadic outbreaks have occurred in other European countries (Belgium in 1985, Netherlands in 1986) and outside of Europe (the Caribbean, Brazil).

Signs Peracute, acute, subacute, and chronic forms of ASF occur, and mortality rates vary from 0 to 100%, depending on the virulence of the virus that pigs are infected with. Acute disease is characterized by a short incubation period of 3–7 days, followed by high fever (up to 42°C) and death in 5–10 days.

The least variable clinical signs are loss of appetite, depression, and recumbency. Other signs include hyperemia of the skin of the ears, abdomen, and legs; respiratory distress; vomiting; bleeding from the nose or rectum; and sometimes diarrhea. Abortion is sometimes the first event seen in an outbreak. Chronic disease is characterized by emaciation, swollen joints, and respiratory problems. This form of the disease is rarely seen in outbreaks.

Human health risk It does not appear that ASF presents a zoonotic risk.

Economic impact The effect on a swine herd can vary depending on the strain, from near 100% mortality to cases of low-virulence isolates that can be difficult to diagnose. When an outbreak occurs in any region or country, the financial and physical implications can be devastating to the swine industry and those related to it.

During outbreaks in Malta and the Dominican Republic, for example, the swine herds of the entire countries were completely depopulated.

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Epidemiology AI, also known as “bird flu,” is a highly contagious viral disease that can infect several species of domesticated birds (chickens, turkeys, quails, guinea fowls, etc.) and wild birds (ducks). It is caused by an A-type influenza virus: a single-stranded RNA virus belonging to the Orthomyxoviidae family. This virus has also been isolated in different mammals, including humans and pigs (the source of genetic recombinations between avian and human viruses).

Signs Avian influenza can present many signs in birds, from minor disease (with little or no clinical signs) to disease that can quickly become fatal and lead to a serious epidemic. Highly pathogenic avian influenza (A virus belonging to H5, H7, and H9 subtypes) is characterized by serious signs and a fast deterioration towards death. The death rate can then reach 100% in less than 2 days. Strains that are highly pathogenic can lead to serious respiratory disease on humans.

Transmission Virus transmission between birds occurs mainly by direct contact (respiratory secretion and fecal material), but can also be indirect (by food or contaminated water, bird droppings carrying the virus, and contaminated materials). Several species of bird are more resistant to the infection than others, such as ducks, which can be infected by pathogenic strains that present unnoticeable clinical signs. The highly pathogenic A/H5N1 strain shows clinical signs in domesticated poultry (chickens and turkeys) and in certain wild birds. The infections caused by highly pathogenic virus strains are rare and must not be confused with the infections caused by less pathogenic strains, which can also belong to the H5, H7, and H9 subtypes.

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Overview CSF is considered the second most serious of all contagious diseases of pigs and wild boars, after aphthous fever. It is a major threat to pig production, with serious socioeconomic consequences.

The disease is caused by an enveloped RNA virus of the genus Pestivirus in the Flaviviridae family. CSF cannot be transmitted to humans and manifests in different ways according to the virulence of the infecting virus and the animal's stage of development.

In all cases, laboratory tests (virology and/or serology) are essential to either confirm or rule out suspected CSF.

Signs A superacute form can cause death within 48 hours with practically no signs, but the more common acute form has an initial phase characterized by high fever (up to 42°C) during which time the animal is lethargic, stops eating, and develops conjunctivitis with a purulent ocular discharge.

The disease also causes gastrointestinal and respiratory problems, hematological imbalance, and neurological disorders. The animal dies within 5–15 days. Some of these signs can be confused with those of many other porcine diseases, which makes CSF difficult to diagnose.

The chronic form of the disease is even more insidious because the signs are mild and infected animals may survive for weeks or months. Moreover, the presence of other concurrent diseases or infections may complicate differential diagnosis.

Economic impact The presence of CSFV in pig herds can have a severe economic impact on the meat production industry as a result of wide-spread animal deaths due to the disease, as well as trade restrictions on meat exports.

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Overview FMD is a highly contagious viral disease that affects all cloven-hoofed animals, and is widespread throughout the world. FMD cannot be differentiated clinically from other vesicular diseases such as swine vesicular disease (SVD).

The virus is a member of the genus Apthovirus in the family Picornaviridae. There are seven serotypes of FMD virus: O, A, C, SAT 1, SAT 2, SAT 3, and Asia 1. Infection with any one serotype does not confer immunity against other serotypes.

Of the domesticated species, cattle, pigs, sheep, and goats are susceptible to FMD.

Signs Clinical signs of FMD are the appearance of vesicles (small blisters) on the feet, in and around the oral cavity, and on the mammary glands of females. Vesicles can also occur inside the nostrils and at pressure points on the limbs, especially in pigs.

Transmission generally occurs through contact between infected and susceptible animals. The virus can be excreted into the air during the acute phase of infection.

Human health risk It does not appear that FMD presents a zoonotic risk.

Economic impact The presence of FMD is a very important economic threat to the livelihood of the livestock industry due to the culling of herds and the restriction on meat exports from affected areas.

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Overview SVD is a highly contagious disease of pigs and the virus causing it (SVDV) is part of the Enterovirus genus in the family Picornaviridae. Although symptoms of the disease are often mild, it is an Office International des Epizooties (OIE) list A disease, as it is clinically indistinguishable from foot and mouth disease (FMD). For this reason, outbreaks of SVD are assumed to be FMD until laboratory diagnosis proves otherwise.

The virus may be spread into the environment through excretions from the nose and mouth and may also be found in the feces.

Signs SVD is typically characterized by the presence of vesicles on the coronary bands, heels of the feet, and occasionally on the lips, tongue, snout, and teats of infected animals.

Human health risk It does not appear that SVDV presents a zoonotic risk.

Economic impact An outbreak of SVD can have a significant economic impact on the meat production industry as a result of the need to cull infected herds to eliminate the disease. Temporary trade restrictions on meat exports are likely to be imposed.

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Overview Transmissible spongiform encephalopathies (TSEs) are infectious diseases of the brain that affect animal species in various forms, such as bovine spongiform encephalopathy (BSE, affecting cattle), scrapie (affecting goats and sheep) and chronic wasting disease (CWD, affecting deer). The diseases are caused by altered prion proteins that are resistant to chemicals and heat, and are very difficult to decompose biologically, often surviving in soil for several years.

The diseases are reported worldwide, with BSE found most frequently in Europe and CWD being most prevalent in North America. TSEs cause a slow degeneration of the central nervous system that ultimately leads to the death of an animal, and there is often a significant lapse of time between an animal becoming infected with the disease and displaying the first symptoms. As an example, at the point of infection, cattle may not show clinical symptoms for up to 6 years, and sheep may not show signs for up to 4 years.

Transmission of BSE in cattle occurs through ingestion of feed containing contaminated bone and meat meal. Transmission does not appear to occur naturally between cattle, though some evidence suggests there may be a maternally associated risk for calves born to infected cows. While pathogenesis details are unknown, studies have shown that after the agent enters the animal through oral exposure, it replicates in the Peyer's patches of the ileum and migrates to the central nervous system via peripheral nerves.

Signs Clinical signs of TSEs are often subtle and may include nervousness, aggression, low head carriage, ataxia, tremors and increased sensitivity to touch (hyperesthesia). Animals may also have a reluctance to be milked and experience weight loss and diminished milk production.

Human health risk Humans can develop a form of TSE known as Variant Creutzfeldt-Jakob disease (vCJD) by consuming food products that have been contaminated with BSE. Initiatives are in place to remove high-risk bovine tissue from the human food chain, and for products containing bovine proteins (cosmetics, pharmaceuticals, etc.), measures have been instituted to help ensure that raw materials are sourced from BSE-free regions.

Economic impact TSEs can contribute to significant economic losses, whether it be culling of animals linked to BSE or scrapie cases, the destruction of Specified Risk Material (SRM) derived from ruminants (cattle, sheep, and goats), or the impact that export restrictions in affected areas may have on the meat industry as a whole.

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